Overview

The ASPTC-L consists of a type-E fine-wire thermocouple mounted in a fan-aspirated radiation shield to provide highly accurate air temperature measurements. You can use one ASPTC to measure absolute air temperature, or you can use two ASPTCs to make delta temperature measurements. Often, the ASPTC replaces, or is measured in addition to, the TCBR thermocouples in a Bowen ratio system.

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Detailed Description

The ASPTC's radiation shield is an elongated tube constructed from white UV-stabilized polyethylene that provides low thermal conductivity and heat retention. A fan draws air across the measurement junction, which reduces solar loading on the thermocouple. The radiation shield also protects the thermocouple, increasing the thermocouple’s durability.

The ASPTC's fine-wire thermocouple consists of a chromel wire and a constantan wire joined at a measurement junction. A voltage potential is generated when the measurement end of the thermocouple is at a different temperature than the reference end of the thermocouple. The magnitude of the voltage potential is related to the temperature difference. Therefore, temperature can be determined by measuring the differences in potential created at the junction of the two wires.

A reference temperature measurement (typically measured at the datalogger wiring panel) is required. Options for measuring the reference temperature include:

Thermistor built into the CR6, CR800, CR850, CR1000, or CR3000 wiring panel

PRT built into the wiring panel of the CR9050 or CR9051E input module for the CR9000X Measurement and Control System

Specifications

Shield Material

UV-stabilized polyethylene

Large Diameter

5.8 cm (2.3 in.)

Small Diameter

4.6 cm (1.8 in.)

Power Cable Diameter

0.5 cm (0.2 in.)

Signal Cable Diameter

0.3 cm (0.1 in.)

Length

53.3 cm (21 in.)

Height

14.7 cm (5.8 in.)

Weight

0.86 kg (1.9 lb)

Fan

Air Velocity at Thermocouple

5.5 m (@ 12 Vdc)

Life Expectancy

65,000 h (@ 30°C)

Current Drain

260 mA (@ 12 Vdc)

Operating Voltage

9 to 13 Vdc

Operating Temperature Range

-10° to +70°C

Polarity Protection

Reverse polarity protected

Thermocouple

Type

Chromel-Constantan

Typical Output

60 μV/°C

Accuracy

Refer to the Thermocouple Measurement section in the datalogger manual.

Diameter

0.0762 mm (0.003 in.)

Compatibility

Note: The following shows notable compatibility information. It is not a comprehensive list of all compatible or incompatible products.

Additional Compatibility Information

Mounting

The ASPTC can be mounted to a tripod or tower via the UT018-5 crossarm or a user-supplied crossarm that has a square cross section of 3.8 cm by 3.8 cm (1.5 inch by 1.5 inch).

Power Considerations

The ASPTC is typically powered with a user-supplied deep-cycle battery that is recharged with an SP20R, SP50, or SP90 solar panel; for high current drain systems, two SP90 solar panels may be used to provide 180 W of power. The datalogger’s rechargeable battery can only be used if it is connected to ac power. For help analyzing your system’s power requirements, refer to our Power Supply brochure or application note.

No. The ASPTC-L was not designed to measure high-frequency temperature fluctuations. Rather, it was designed to measure the mean air temperature or mean air temperature gradients. To find sensible heat flux using the eddy-covariance technique, consider using a sonic anemometer and an FW05 Fine Wire Thermocouple.

Design the station power supply to meet the power demand of the ASPTC-L fan(s). The data acquisition system should have provisions to turn off the ASPTC-L fan(s) if the battery voltage falls below some threshold. Do not turn the fan(s) off at night during normal operation, however, because under clear sky conditions, the ASPTC-L shield emits long-wave radiation and cools to a temperature lower than ambient air temperature. Running the fan(s) ensures that the thermocouple is measuring ambient air temperature, rather than a reduced temperature caused by long-wave radiation cooling.

No. Replacing the thermocouple requires completely disassembling the fan and shield. It is best to replace the ASPTC-L with a spare and send the broken one back to the factory for repair and cleaning. To request a returned material authorization (RMA) number, follow the steps listed on our Repair and Calibration page. Request pn 16537, ASPTC-L Cleaning and Repair Fee.

No. If the fan is off during the day, the shield will suffer from radiation loading, and the thermocouple will measure a higher ambient air temperature. If the fan is off during the night, under clear sky conditions, the ASPTC-L shield will emit long-wave radiation and cool to a temperature lower than ambient air temperature. Consequently, the thermocouple will measure a lower ambient air temperature. Running the fan and aspirating the thermocouple ensures that it is measuring ambient air temperature.